The present invention relates to rotary blowers of the Roots-type, and more particularly, to such blowers of the backflow-type.
As is well-known, Roots-type blowers include lobed rotors meshingly disposed in transversely overlapping cylindrical chambers defined by a housing. Spaces between adjacent unmeshed lobes of each rotor transfer volumes of air from an inlet port opening to an outlet port opening, without mechanical compression of the air in each space.
Typically, Roots-type blowers of the type described above are used as superchargers for vehicle engines wherein the engine provides the mechanical torque input to drive the lobed rotors. The volumes of air transferred to the outlet port are then utilized to provide a pressure "boost" within the intake manifold of the vehicle engine, in a manner which is well known to those skilled in the art, and is not directly relevant to the present invention.
Among the criteria used in evaluating a Roots blower supercharger are the horsepower required to transfer a particular volume of air under certain operating conditions, and the extent to which the temperature of the air being transferred increases as it flows through the supercharger. Such increases in the temperature of the air being transferred are reflected in a decrease in the isentropic efficiency (also referred to as the adiabatic efficiency) as will be described in greater detail subsequently.
The horsepower required to drive the supercharger is clearly understood to represent a horsepower "loss", from the viewpoint of the horsepower output of the engine, and therefore, it is desirable to minimize the required drive horsepower. In the case of the increase in air temperature as the air is transferred through the supercharger, such temperature increase also represents a horsepower loss. Typically, vehicles equipped with accessories such as superchargers have included an intercooler, one function of which is to cool the air which is being transferred by the supercharger to the engine. The warmer the air passing through the intercooler, the more horsepower is consumed by the intercooler in bringing the air temperature down to the desired temperature for optimum engine efficiency.
Much development has been done by the assignee of the present invention to improve the volumetric efficiency of Roots blower superchargers. In particular, the development has focused upon improving the configuration of the outlet port and the inlet port, as is illustrated and described in U.S. Pat. Nos. 4,768,934 and 5,078,583, respectively. However, even greater improvements in supercharger efficiency have been sought, and in particular, there has been much effort to increase the isentropic efficiency of the supercharger. It has been recognized that, in some vehicle applications, a substantial increase in isentropic efficiency of the supercharger could make it possible to eliminate the intercooler, which would represent a major cost saving for the vehicle OEM (original equipment manufacturer).